The present invention relates to magnetically coupled resonant structures, and in particular to tracking the positions and/or orientations of such structures.
Radio frequency identification (RFID) devices have been employed for some time to remotely sense parameters of interest in people or objects. An RFID device or xe2x80x9ctagxe2x80x9d receives a wireless signal from an externally located xe2x80x9creader,xe2x80x9d which determines the identity (or other parameter of interest) of the item based on the response of the RFID device to the transmitted signal.
For example, the tag may contain an inductor and capacitor arranged in a parallel LC configuration so as to exhibit a characteristic resonant frequency. In this case, the transmitted signal may be a time-varying magnetic field produced by a xe2x80x9csearchxe2x80x9d coil. This interrogation signal is pulsed at specific frequencies or swept through a range of frequencies and interacts with proximately located RFID tags. A tag is detected when the frequency of the interrogation field reaches the resonant frequency of the LC tag. Detection may take place in either of two ways: by using a separate receiving coil to measure the signal from the search coil, which will change at the tag""s resonant frequency because of perturbations to the magnetic coupling; or by measuring the loading on the search coil, which will increase at this frequency as the tag extracts energy from the search coil""s field.
While such systems are well-suited to detection and identification (based on the resonant frequency) of magnetically coupled tags, localizing the position of a detected tag is much more difficult: the magnitude of the detected signal depends not only on the tag""s distance from the coil, but also on its orientation and position with respect to the coil. This is because of the shape of the field; the coupling is a function of the projection of the local magnetic field vector onto the tag""s magnetic axis. The magnetic field of a coil is essentially toroidal, extending from one end of the coil and looping around to the other end in a radially symmetric fashion. As a result, the field is substantially uniform and directed along the axis of the coil only within the coil and in close proximity to its (axial) ends. Outside this region, the interaction between the tag and the field will be strongly dependent on tag orientation and position with respect to the field curvature.
Brief Summary of the Invention
In accordance with the present invention, a structure capable of magnetic coupling is tracked using one or more pairs of coils oriented such that, when the coils are energized, a substantially uniform magnetic field is created in a region between the coils. The field magnetically couples into any appropriately aligned structures located in the region between the coils. A value for a signal parameter indicative of the coupling is obtained for each coil; for example, that parameter may be the degree of loading on the coil driver. From the signal-parameter values, the position of the structure may be deduced. Each pair of coils provides position information along their common axis. By arranging multiple pairs of coils in orthogonal relation to each other, multiple-axis position information can be obtained. If six such measurements are taken (e.g., search-coil loading for each coil of three orthogonal pairs), the three translational positions and three absolute rotational inclinations (|xcex1|, |xcex2|, |xcex3|) can be determined.
If the tag""s magnetic axis is exactly orthogonal to the magnetic field axis generated by a pair of search coils, however, it will not couple so as to facilitate detection by either search coil. Full tracking can still be achieved, however, in any of various ways:
a) Constraining the inclination range of the tag such that it never becomes orthogonal to the search coil fields.
b) Using a plurality of non-aligned magnetic resonance elements, such as an aggregate of three tags in an orthogonal triad, for example.
c) Coupling magnetic flux from all three axes into a single resonant structure or circuit.
d) Adding additional search coils to span intermediate axes.
e) Driving non-aligned search coils to produce off-axis magnetic fields that project between the orthogonal axes to reduce ambiguities.
f) Using a single- or double-axis tag in conjunction with a tracking filter or algorithm that interpolates the tag position from the point at which a coordinate disappears (e.g., the tag orientation becomes orthogonal) and reappears again.
The invention is amenable to a wide range of applications. An LC resonance tag may be made small, lightweight, and low in cost; these characteristics, in combination with nonreliance on battery power, enable virtually any object to be xe2x80x9ctaggedxe2x80x9d and to thereby serve as an interface device. An LC tag or combination of them may, for example, be worn on a finger or hand placed within the sensing volume 200 as part of an entertainment device such as a game, a virtual-reality environment, or a music synthesizer, with the detected (and frequently updated) three-dimensional position of the tag used as input. Three-dimensional tracking can also be useful in medical contexts, e.g., to follow the position of a tagged physical location during a procedure. For example, radiologists frequently must irradiate a volumetric region larger than a tumor under treatment in order to accommodate patient breathing or other movement. This results in otherwise unnecessary damage to healthy tissue. In accordance with the present invention, a small tag may be surgically associated with the tumor and its location monitored in three dimensions, thereby facilitating automatic steering of the treatment beam to follow the tumor as it moves. The system can detect and track multiple tags simultaneously, assuming that their resonant frequencies do not overlap.